The invention relates to a method for bringing about a fuel oxidation in a recirculating exhaust gas stream for heating a volume, in particular at temperatures of the volume below the ignition temperature of the fuel, and also to a burner for achieving said method.
EP 0 685 683 B1 discloses that volumes such as, for example, furnace spaces or the interior spaces of jet pipes can be heated by flameless oxidation of fuel. In order to bring about flameless oxidation, a strong recirculation of the combustion gases is brought about in the affected volume. Preheated air and fuel are injected into the combustion gases. In so doing, a uniform release of thermal energy occurs in a larger section of volume with minimal thermal NOx formation at a reaction temperature of below 1500° C. However, this requires that sufficiently high temperatures prevail in the furnace space. In accordance with EP 0 685 683 B1, as well as in accordance with WO 2007/017056 A1, this is accomplished by means of a combustion chamber which is integrated into the burner, whereby fuel as well as combustion air can be injected into said combustion chamber. The resultant mixture is ignited in the combustion chamber. The developing hot gases leave the combustion chamber and arrive in the volume that is to be heated. Such burners are set up for two modes of operation:
1. Combustion with a stable flame in the combustion chamber as long as the temperature in the volume to be heated is below the ignition temperature of the fuel that is being used.
2. Combustion outside the combustion chamber in the volume to be heated as soon as the temperature of said volume is above the ignition temperature. The high exit speed of the air effects the desired strong recirculation of the combustion gases in the volume, thus preventing high temperatures and the thermal NOx formation.
There are applications in which the volume to be heated can or should be heated only gradually, for example, due to its size or the character of its content. It is also possible that the desired process-specific final temperature is relatively low, for example, below the ignition temperature of the fuel.
If, in these cases, the furnace space is heated by means of a flame, high temperatures occur locally (in and around the flame), thus resulting in the formation of NOx in the exhaust gas.
It is the object of the present invention to reduce the NOx formation in the heated space also at those temperatures which are below the ignition temperature of the fuel used.